What is "Read Render Layer" actually for?
Quite frankly never heard of that before.
According to the Blender manual it is used to read cached render buffers from disk, possibly after writting them to save memory
From the Python API manual

bpy.ops.image.read_renderlayers()
Read all the current scene’s render layers from cache, as needed

It is probably used with the Save Buffers option under the Performance tab of the Render panel in the Properties Window which according to the manual:

Saves all render layers and passes to the temp directory on a drive,
and read them back after rendering has finished. This saves memory
usage during rendering, particularly when using many render layers and
passes.

Haven't figured out how to restore image after using it, I'm afraid it is lost, though I don't really fully understand the purpose of this nor how to use it.